Rotted is a minecraft mod that adds new types of zombies, as well as revamping the already existing ones, making them drop custom loot and bonding them to have special AI. Rotted Zombies take more damage from fire than normal ones do. But, they are "pure" zombies, making them infect other creatures with the "contagion" effect with a 80% chance, whilst the normal zombies only have a 20% chance. When a player or villager dies with this effect, it's zombified counterpart will spawn. The counterpart of the player also gets some of his armor, his weapon and name.
And if the zombies may seem to strong for you, you still have the config file to work with, playing with their health and damage! Rotted looks beautifully with shaders and with other mods because this mod has huge compatibility with other mods even if, some of the shaders or mods may break the actual visual / game effect the zombies present.
Also very important! If you interchange versions that add mobs, you have to actually add those mobs in the rottedconfig.toml file, found in the .minecraft folder or any config thingy. You have examples on how to add them with a comma and " sign.